The changing climate
The earth's climate system is chaotic in the sense that it is continuously changing. Temperature on our planet has changed much and quickly before. These changes can be global or regional: a temperature increase in the northern hemisphere can have had a corresponding lower temperature or no temperature change in the southern part of the world. A "little ice age" in the area around the North Atlantic, has not necessarily affected other regions of the world. A warm 'bronze age' in Northern Europe 5000 years ago did not necessarily change global mean temperatures much: an extra tilting angle of the polar axis would entail a warmer north but a colder south. As a rule, solar forcing, negative or positive, has been the main driving or radiative forcing agent for significant climate change throughout the times. Albedo and the chemical composition of the atmosphere have been feedback mechanisms, enhancing the solar forcing either way.
  1. The global temperature may vary due to solar activity.
    Most researchers think that the temperature rise during the first part of last century was connected to increased solar radiation, and some voice the opinion that at least parts of the temperature rise the last decades also has been caused by increased solar activity.

    However, according to the UN Panel of Climate Change (IPCC), it is unlikely that the temperature rise during the last decades can be explained by increased solar activity alone.
    The last forty years, every decade has been warmer than the preceding decade.
    This in spite of that solar activity this period has been the lowest for hundred years except for a solar outburst in 1998.
    In addition, large amounts of aerosols from coalfired powerplants and increasing traffic in the air, on sea and land camouflage parts of the warming trend.

    World warmer, but trends at odds: report (ABC Science)

    Solas variabilitet og klimaet (CICERO)
  1. Solar forcing
    A change in the solar radiation having an effect on the earth's climate is called "solar forcing".
    When the solar radiation is less than usual, the forcing is said to be negative.
    When the sun warms the earth more than usual, the forcing is positive.
    Solar forcing tends to vary in fairly regular cycles.

    Radiative forcing
    Radiative forcing can in a simple way be defined as a force that changes the equilibrium of the energy budget in our climate system.
    Solar activity, one of the radiative forcing factors, varies, and the solar forcing can be quite significant one year, down to "equilibrium"; i.e. no forcing the next.
  1. Aerosols
    Greenhouse gases also have a forcing effect on our climate systems.
    Increasing amounts of greenhouse gases in the atmosphere have a positive forcing effect.
    In contrast to solar radiation, greenhouse gases can accumulate; additional amounts of these gases do not disappear overnight, but can stay in the atmosphere for decades and in some cases for centuries.

    Negative albedo forcing from particles (aerosols) in the air, e.g. from burning of fossil fuels, will last for some hours or days.
    Sulphur aerosols from big volcanic eruptions may stay in the stratosphere for 1-4 years, having a significant cooling effect in that period.

    While solar forcing tends to level out over time, the forcing from additional, anthropogenic greenhouse gases in the atmosphere will increase over time.
    Radiative forcing is therefore often calculated over e.g. a 100-year period.

    Climate change due to changes in the atmospheric chemistry will therefore be more enduring than those caused by changes in solar forcing.
    Whether the radiative forcing is caused by solar activity or changes in the atmospheric chemistry or not, there will be an impact on the third main factor with influence on the world's climate: albedo.

    A cold climate over a long period will increase the size of ice covers, entailing an increase in the albedo, meaning that more sunlight will be reflected and thus strengthen the cooling process.
    Increasing albedo means increasing negative radiative forcing.
    Cooling activates feedback mechanisms enhancing the cooling, and vice versa: warming will activate feedback processes enhancing the warming.

    Graphic: UNEP/GRID-Arendal, Vital Climate Graphics, 2001